1. Field of the Invention
The present invention relates to a semiconductor device having an insulated gate structure using a crystalline silicon film disposed on an insulating substrate of glass, quartz or the like, for example, a thin film transistor (TFT), a thin film diode (TFD), and a thin film integrated circuit using the thin film elements, especially a thin film integrated circuit for a passive matrix type liquid crystal display device and a thin film integrated circuit for an active matrix type liquid crystal display device, and also relates to a method of manufacturing the same.
2. Description of the Related Art
In recent years, there has been vigorously carried out studies on an active matrix type liquid crystal display device in which thin film transistors are formed in a matrix form on an insulating substrate of glass, quartz or the like, and the TFTs are used as switching elements.
Also, an attention has been paid to an active matrix type liquid crystal display device in which an active matrix circuit (also called a pixel circuit or pixel matrix circuit) and a peripheral drive circuit (also called a driver circuit) are integrated on the same insulating substrate. This structure is called a peripheral drive circuit integration type.
A conventional active matrix type liquid crystal display device uses transparent electrodes in which electrodes for driving a liquid crystal layer are formed on two substrates faced to each other. A liquid crystal is sealed between the two substrates, and the direction of an electric field applied to the liquid crystal is made substantially perpendicular to the surface of the substrate. The liquid crystal device is realized by changing the intensity of the electric field so that the direction of orientation of liquid crystal molecules generally having a rod shape are changed so as to be parallel to the substrate or perpendicular to the substrate. Generally, in this case, since light is made to be modulated by using an optical anisotropy as one of features of the liquid crystal material, a polarizing plate is disposed in the device so that incident light is made linearly polarized light.
However, in the liquid crystal electro-optical device having such an operation method, there is observed such a phenomenon that although the state of display is normal when the display surface is seen from the direction perpendicular thereto, the display is dark and becomes blurred when the display surface is seen from the direction at an angle thereto, and further, color is changed when the display is colored.
In order to solve such a problem, there is a method (IPS mode) in which the direction of an electric field applied to a liquid crystal layer is made parallel to the surface of a substrate.
In such an electro-optical device, since switching is carried out while a longitudinal axis of a liquid crystal molecule is kept parallel to the substrates, the change of optical characteristics of the liquid crystal due to an angle of visual field is small.
Thus, leak of light, lowering of contrast and the like due to the angle of visual field are smaller than a conventional TN or STN system.
There is known a structure of an electrode of this IPS mode as shown in FIG. 17 in which a comb-shaped electrode is formed on one substrate.
However, there is a problem that when the comb-shaped electrode is used, a wiring pattern is made minute and complicated in a pixel element, so that productivity becomes inferior.
Also, since the shape of the electrode is complicated, an electric field applied to a liquid crystal layer becomes complicated.
Further, light is shaded by the comb-shaped electrode, so that an effective area (opening rate) through which light can passes, becomes extremely low. Thus, only a dark display can be realized, and it can not be put into practical use.